Active cover accelerometer

ABSTRACT

A pendulous accelerometer wherein the active reaction mass is pendulously mounted external to a fixed support structure and may include sensor cover or covers in the total active reaction mass.

This application claims priority from U.S. provisional application Ser.No. 60/044,034 filed Apr. 22, 1997, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The invention relates to acceleration sensors, and in particular to areaction mass used with a pendulous accelerometer.

Pendulous accelerometers, for example, vibrating beam accelerometers,capacitive accelerometers, capacitive rebalance accelerometers, andtranslational mass accelerometers comprise a reaction mass. Existingdesign and manufacturing techniques for these devices are described inU.S. Pat. No. 4,495,815 "Mass And Coil Arrangement For Use In AnAccelerometer, " U.S. Pat. No. 5,396,798 "Mechanical Resonance, SiliconAccelerometer, " U.S. Pat. No. 4,766,768 "Accelerometer With IsolatorFor Common Mode Inputs, " U.S. Pat. No. 5,228,341 "CapacitiveAcceleration Detector Having Reduced Mass Portion, " U.S. Pat. No.5,350,189 "Capacitance Type Accelerometer For Air Bag System, " U.S.Pat. No. 4,335,611 "Accelerometer, " and U.S. Pat. No. 3,702,073"Accelerometer " which are incorporated herein by reference.

All practical pendulous accelerometers to date function on the principleof Neuton's law that force equals mass times acceleration. In manyaccelerometer applications high performance and small size aredesirable. One problem with the design of small, high performancependulous accelerometer sensors involves obtaining adequate reactionmass in a small space. A second problem with the design of small, highperformance pendulous accelerometer sensors involves providing adequateisolation from the mounting structure such that mounting strains do notaffect accelerometer performance.

Typical accelerometer sensors include a pendulous reaction mass, oftenreferred to as a proof mass, suspended from a stationary frame by, forexample, a flexural suspension member or some other form of pivotmechanism. This pivot constrains the reaction mass to only one directionof motion; the reaction mass is free to move along this one direction ofmotion unless restrained to the null position. The pendulous reactionmass must be restrained under acceleration by an opposing force whichmay be the result of a position feedback circuit. Alternatively, theaccelerometer may be an open-loop device in which the opposing force maybe supplied a spring in the form of, for example, pivot stiffness.

In a typical accelerometer sensor mechanism the pendulous reaction massis suspended on a flexural suspension member inside an external supportframe. Isolation is typically provided by mounting the supporting frameitself inside an isolation feature supported from a final exterior framewhich provides mounting both to sensor covers and to the accelerometerhousing. The above features as practiced in a typical vibrating beamaccelerometer sensor are shown in FIGS. 1 and 2. The large exteriorframe system is static and adds no mass to the active reaction mass.Additionally, any external strain couples through the exterior framesystem directly across the length of the sensor mechanism. The resultinglarge frame dimensions tend to maximize the effect of error drivers, forexample, thermal expansion mismatch, placing additional burden on theisolator function.

SUMMARY OF THE INVENTION

The present invention resolves significant problems of the prior art byproviding both superior mounting stress isolation and substantiallyreduced acceleration sensor mechanism size while maintaining adequatemass in the reaction mass without increasing manufacturing costs. In thepresent invention the external frame isolation system is eliminated andthe remaining structure becomes the active reaction mass. The presentinvention describes various embodiments optimized for various Orangeapplications. The illustrated embodiments substantially reduce mechanismsize and maximize active mass while maximizing isolation from externalerror sources and minimizing heat flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one prior art device using vibrating beam technology;

FIG. 2 is a cross-sectional view of the device illustrated in FIG. 1taken along section line A--A;

FIG. 3 illustrates an internal mount acceleration sensor mechanismaccording to one embodiment of the present invention;

FIG. 4 illustrates an active cover acceleration sensor mechanismaccording to another embodiment of the present invention; and

FIG. 5 illustrates an internal mount acceleration sensor mechanismaccording to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention resolves significant problems of the prior art byproviding both superior mounting stress isolation and substantiallyreduced acceleration sensor mechanism size while maintaining adequatemass in the reaction mass without increasing manufacturing costs.

An accelerometer sensor may include pendulous reaction mass, oftenreferred to as a proof mass, suspended from a stationary frame by, forexample, a flexural suspension member or some other form of pivotmechanism. This pivot constrains the reaction mass to travel along onlyone axis unless the reaction mass is restrained to the null position.

FIGS. 1 and 2 illustrate a typical vibrating beam acceleration sensormechanism 10 having a pendulous reaction mass 12 suspended on a flexuralsuspension member 14 inside a first external support frame 16. Supportframe 16 itself is typically mounted inside an isolation featuresuspended from a final exterior frame 18 that provides mounting both fortop cover 20 and bottom cover 22. Typically, sensor 10 is mounted withinthe accelerometer housing 24, represented by ground, by fixing bottomcover 22 to accelerometer housing 24. Alternatively, the sensorpackaging is reconfigured such that sensor 10 is mounted by attachingtop cover 20 to accelerometer housing 24. In the typical accelerometerof FIGS. 1 and 2, exterior frame 18 is much larger than reaction mass 12and larger than external support frame 16. In operation the relativelylarge exterior frame system comprising support frame 16 and externalframe 18 remains static relative to the pendulous reaction mass. Thus,support frame 16 and external frame 18 add no reaction mass to activereaction mass 12.

Reaction mass 12 is free to move relative to support frame 16. However,reaction mass 12 is typically constrained to movement along an inputaxis 26 aligned substantially perpendicular to the plane of reactionmass 12. Thus, an input force, for example, an acceleration input,applied along input axis 26 displaces reaction mass 12 a distance, x,along input axis 26. Vibrating beam force sensors 28, 30 are mounted toextend between central support frame 16 and reaction mass 12 such thatdisplacement of reaction mass 12 relative to support frame 16 impartseither a compressive or a tensile force to force sensors 28, 30.

External strains experienced by the accelerometer housing may affectacceleration sensor performance. External strains may be caused by, forexample, mismatch of thermal expansion coefficients between structuralmembers, physical distortions of housing mounts due to clamping forces,or shocks and vibrations experienced by the housing. In the exampleillustrated in FIGS. 1 and 2, external strains experienced byaccelerometer housing 24 are transmitted to external frame 18 throughthe structure connecting external frame 18 to accelerometer housing 24,for example, through bottom cover 22. A typical design providesisolation between vibrating beam force sensors 28, 30 and externalstrains experienced by external frame 18. The isolation feature maycomprise, for example, isolation slots 32, 34. Although vibrating beamforce sensors 28, 30 and reaction mass 12 are isolated from externalframe 18 by an isolation feature, for example, isolation slots 32, 34,external strain experienced by accelerometer housing 24 may couplethrough exterior frame 18 and the isolation features directly across thelength of sensor mechanism 10. Additionally, the large dimensions ofexternal frame 18 tend to maximize the effect of error drivers, forexample, thermal expansion mismatch, placing an additional burden on theisolator function.

One embodiment according to the present invention as practiced in avibrating beam acceleration sensor is illustrated in FIG. 3 by invertingthe typical accelerometer mounting system. The acceleration sensormechanism 40 of FIG. 3 provides an internal mount/external reaction massconfiguration wherein the external frame isolation system is eliminatedand the remaining structure becomes the active reaction mass. Accordingto the embodiment illustrated in FIG. 3, reaction mass 12 is replaced byan internal frame member 42 sandwiched between top cover 44 and bottomcover 46 wherein each cover 44, 46 is formed with a central pedestalportion 48, 50, respectively. Central pedestal portions 48, 50 arebonded to opposing surfaces of internal frame member 42 using a suitablebonding method, for example, epoxy bonding. In one embodiment of thepresent invention, bottom cover 46 is mounted to an accelerometerhousing 52 represented by ground. Thus, internal frame member 42 andcovers 44, 46 are combined to form a single frame structure fixed toaccelerometer housing 52. Alternatively, acceleration sensor 40 ismounted by fixing top cover 44 to accelerometer housing 52. According tothe embodiment illustrated in FIG. 3, an external reaction mass 54 isdisposed around and external to internal frame member 42 and rotatablysuspended therefrom by, for example, a flexural suspension member 14 oranother suitable form of pivot mechanism. External reaction mass 54 isformed with an internal passage through its thickness suitable to nestinternal frame member 42 rotatably within the passage. Reaction mass 54and internal frame member 42 may be formed of a single piece ofsubstantially planar substrate material, for example, quartz or silicon,using, for example, laser cutting technology or other manufacturingtechniques known to those of skill in the art. Flexural suspensionmember 14 may also be formed in the single substrate. Reaction mass 54is constrained, for example, by the hinge mechanism or other means, tomovement along an input axis 56 substantially perpendicular to the planeof the substrate. Thus, an input force, for example, an accelerationinput, applied along input axis 56 displaces reaction mass 54 adistance, x, along input axis 56. Vibrating beam force sensors 28, 30are mounted to extend between internal frame member 42 and reaction mass54 such that displacement of reaction mass 54 relative to internal framemember 42 imparts either a compressive or a tensile force to forcesensors 28, 30.

The present invention also results in increased isolation from bothexternal stresses and mounting stresses by providing localized straincoupling instead of multiplying external strain coupling across thelength of the mechanism. According to the embodiment illustrated in FIG.3, the isolation function of external isolation features, for example,isolation slots 32, 34, is obviated. Rather, the isolation function isperformed by central pedestals 48, 50. Central pedestals 48, 50 isolatethe sensor mechanism from external strains by reducing the interfacearea to a minimum and by placing the interface point at the center ofsensor mechanism 40 such that the moment arm over which anystrain-induced force acts is also reduced to a minimum. Thus, stressmagnitude is minimized and constrained to a small locality. The stressedlocality is nearly ideal because it is centrally located and symmetricalrelative to the vibrating beam force sensors.

Strain-induced forces and forces developed at the interface betweencentral pedestals 48, 50 and internal frame member 42 may be furtherreduced by fixing pedestals 48, 50 to internal frame member 42 using acompliant epoxy bonding technique, for example, the techniques disclosedin U.S. Pat. No. 5,532,665 or U.S. patent application Ser. No.08/665,036 entitled "Low Stress Magnet Interface " and assigned to theassignee of the present patent application, both of which areincorporated herein by reference, or by conventional epoxy bondingtechniques known to those of skill in the art. When the isolationfunction is thus obviated, the volume previously occupied by centralsupport frame 16 and external frame 18 is available to accommodate alarger active reaction mass without increasing the overall accelerationsensor dimensions.

Additionally, the embodiment of the present invention illustrated inFIG. 3 provides an increased pendulous arm length, the distance from thepivot point of flexural suspension member 14 to the tip of reaction mass54, by increasing the dimensions of active reaction mass 54 andpositioning active reaction mass 54 external to internal frame member42. Thus, the available mass is used more efficiently.

Alternatively, the present invention according to the embodimentillustrated in FIG. 3 may be scaled down to use a reaction mass 54 whichoccupies less than the volume formerly occupied by support frame 16 andexternal frame 18. Thus, an acceleration sensor having an activereaction mass commensurate with that of the configuration illustrated inFIGS. 1 and 2 occupies less volume than if configured according totypical sensor designs. Those of skill in the art will recognize thatpresent invention provides an opportunity to trade volume forpendulousity and vice versa along a continuum ranging from maintainingthe original active reaction mass in a reduced volume sensor toincreasing the active reaction mass to fill the original volume. Thoseof skill in the art will further recognize that the degree to whichvolume is traded for active reaction mass is dependent on many designfactors including, for example, available space in which to mount theacceleration sensor, the grange required by the application, and thematerials used in manufacturing the sensor.

The embodiment of the present invention illustrated in FIG. 3 furtherprovides improved thermal response. The heat capacity of suspendedreaction mass 54 is lower because the internal mount/external reactionmass configuration provides greater pendulousity using less totalpendulous mass. Thus, the raw thermal response time is quicker due tothe higher mass efficiency of the reaction mass. Further thermalresponse aid results from the centrally localized mounting. Because theheat source is centrally localized, heat flow paths are simplified. Theheat flow paths are nearly symmetrical and easily controlled byconventional methods which allows more precise thermal ramp matching.According to the embodiment shown in FIG. 3, the heat flow path betweenthe heat source at housing 52 to the detector elements, force sensors28, 30, begins at the interface between housing 52 and bottom cover 46and is channeled into internal frame member 42 through pedestal portion50 which is the single point interface between bottom cover 46 andinternal frame member 42. Although external reaction mass 54 may presentsome secondary effects, the dominant heat flow path between pedestalportion 50 and force sensors 28, 30 is through internal frame member 42.Thus, the heat flow is easily modeled for computer analysis. Incontrast, the heat flow path of the prior art embodiment shown in FIGS.1 and 2 is far more complex and difficult to model. The heat flow pathof the prior art embodiment shown in FIGS. 1 and 2 is through bottomcover 22 into exterior frame 18 via peripheral interface 36(cross-hatched area of exterior frame 18 shown in FIG. 1) between bottomcover 22 and exterior frame 18. The heat flow analysis is furthercomplicated by heat flow from exterior frame 18 into top cover 20 atinterface 36. The heat flow path continues into support frame 16 throughthe isolation feature and through the irregular shape of support frame16 to force sensors 28, 30. Secondary effects are presented by heat flowthrough hinge 14 into pendulous reaction mass 12 and through reactionmass 12 into force sensors 28, 30. According to the embodiment of FIG.3, the improved heat flow paths combined with the reduced heat capacityin the reaction mass produces smaller, more quickly dissipatedheat-induced errors when compared with the prior art embodiment shown inFIGS. 1 and 2.

Isolation features may be added around central mounting pedestals 48, 50to further decouple stress and control heat flow. The isolation featuresmay be, for example, similar to eliminated isolation slots 32, 34between support frame 16 and external frame isolator 18 or otherisolation designs known to those of skill in the art. Additionally,isolation features may be much smaller than disclosed in the prior artand still prove more effective against the lower initial stress levelsbecause the internal mount/external reaction mass configurationdecouples stress and controls heat flow. Thus, simple isolationfeatures, for example, isolation slots 32, 34, a "C" shaped slot, a pairof "()" shaped slots or other isolation features known to those of skillin the art will both dissipate stress and direct heat flow. However,isolation features are outside the scope of this invention and are notshown.

Practical accelerometers also generally require a source of reactionmass damping and shock stops for the reaction mass. Typicalaccelerometer sensors provide shock stops to keep the motion of thereaction mass from over stressing the flexures and, in a vibrating beamaccelerometer such as depicted in FIG. 1, from over stressing thevibrating beam force sensors when the reaction mass experiences largeacceleration inputs. The shock stops typically comprise physicalconstraints which limit the motion of the reaction mass along the inputaxis. The reaction mass shock stop function is often obtained bylocating the reaction mass between two stationary cover plates. In theconfiguration depicted in FIG. 1, top cover 20 and bottom cover 22provide the shock stops for reaction mass 12. The motion of reactionmass 12 is limited by actual physical contact with either top cover 20or bottom cover 22.

According to the embodiment of the present invention as illustrated inFIG. 3, shock stops are provided by top and bottom covers 44, 46. Themotion of reaction mass 54 is physically limited by the proximity ofcovers 44, 46. Any displacement, rotational or translational, ofreaction mass 54 along input axis 56 is restricted by physical contactwith either top cover 44 or bottom cover 46.

Often, viscous gas damping of the reaction mass is desirable andsometimes necessary to avoid the effects of undesirable reaction massoscillations which may deteriorate sensor performance. Gas dampingtypically involves locating a pressurized fluid, for example, oil, airor a mixture of inert gases such as nitrogen and helium, in contact withthe reaction mass, thereby controlling the effects of an oscillationwhich would deteriorate the performance of the device. For example, ashock or vibration input force can cause the reaction mass to oscillateback and forth about its rest position after the force has been removedfrom the reaction mass. Undesirable oscillations can also be caused byvibrations in the surrounding structure. The reaction mass dampingfunction may be obtained by sandwiching the reaction mass between thetwo stationary cover plates and introducing fluid or gas pressure intothe chamber surrounding the reaction mass. Typically, the gas pressureis substantially above vacuum pressure, for example, on the order of outone atmosphere. In a gas-filled mechanism, the closely spaced coverplates constrain the gas such that squeeze film effects provide dampingof the reaction mass along the input axis. In a traditionalconfiguration the cover plates are bonded directly to the mechanismframe for accurate control of the shock and damping gap spacing. In theexample of FIGS. 1 and 2, cover plates 20, 22 are bonded to thecross-hatched area of external frame 18 at bond lines 36. Pressurizedgas is introduced into sensor mechanism 10 and trapped in the chambersurrounding reaction mass 12. Thus, as reaction mass 12 attempts amotion toward top cover 20 during oscillation, the pressurized gastrapped between reaction mass 12 and top cover 20 is squeezed, and whenreaction mass 12 attempts a motion toward bottom cover 22, the trappedpressurized gas is squeezed between reaction mass 12 and bottom cover22. Thus, oscillations of reaction mass 12 are damped by the resistanceof the pressurized gas to being further compressed or squeezed.

The traditional configuration, for example, the configuration of FIGS. 1and 2, locates most of the mechanism mass statically relative to theaccelerometer housing such that it contributes no mass to the activereaction mass. As accelerometer size is reduced or scaled down, theactive or useful mass of the reaction mass is reduced. Therefore theforce available to be sensed for a given acceleration is reduced inproportion to the reduction in reaction mass. As the available forceshrinks, the significance of the noise level for any given sensingmechanism increases, limiting the resolution and accuracy of theaccelerometer. In other words, the signal-to-noise ratio decreases withdecreasing reaction mass and accelerometer resolution and accuracy arereduced proportionally. Thus, size reduction through scaling is limitedby the necessity of maintaining a minimum significant amount of activereaction mass. While the embodiment of the invention shown in FIG. 3provides greater reaction mass efficiency than typical sensor designsand is a preferred embodiment for some applications, other applications,for example, lower g-range applications requiring accurate sensing ofaccelerations in the micro-g range, may benefit from even greaterreaction mass efficiency.

FIGS. 4 and 5 illustrate two further embodiments of the presentinvention optimized for low g range applications. For example, anaccelerometer using either of the embodiments illustrated in FIGS. 4 and5 may be used to measure accelerations in the micros range. Theembodiments illustrated in FIGS. 4 and 5 provide substantially reducedmechanism size and maximum active reaction mass. In other words, theembodiments of FIGS. 4 and 5 provide maximum signal-to-noise ratio in aminimum size mechanism. In contrast to typical sensor designs, in FIG. 4the arrangement of the reaction mass, frame and covers is altered suchthat the cover plates are instead bonded to the moving pendulum itselfadding their mass to the active reaction mass. The resulting mechanismcan be mounted by its frame such that the cover plates now movedynamically with the reaction mass and add their mass into the activereaction mass. In sensor 60, frame 62 is formed with an internal passagethrough its thickness wherein internal pendulum portion 64 is nested.Internal pendulum portion 64 is suspended by flexure 66 from surroundingframe 62. Some or all of frame 62, internal pendulum 64 and flexuralsuspension member 66 may be formed of a single substantially planarsubstrate using manufacturing techniques known to those of skill in theart. The substrate material may be, for example, quartz, silicon oranother suitable material. Frame 62 may include an isolation feature,for example, conventional isolation slots which divide frame 62 into aninner support frame and an external mounting frame. Vibrating beam forcesensors 28, 30 extend between internal pendulum 64 and frame 62 suchthat displacement of internal pendulum 64 imparts either a compressiveor a tensile force to vibrating beam force sensors 28, 30.

The mechanism of FIG. 4 overcomes the traditional limitations ofconventional sensor designs by providing substantially reduced mechanismsize and maximum active mass. In FIG. 4, top cover 68 and bottom cover70 are formed with pedestal portions 72, 74, respectively. Top cover 68and bottom cover 70 are bonded to opposing surfaces of internal pendulum64. For example, top and bottom covers 68, 70 may be bonded to internalpendulum 64 at the geometric center of internal pendulum 64, at thecenter of mass of internal pendulum 64 or at the center of percussion ofinternal pendulum 64. Thus, top cover 68 and bottom cover 70 add theirmass to the active mass of internal pendulum 64 which moves essentialreaction mass to internal pendulum 64 and maximizes the active reactionmass without increasing sensor dimensions. The impact of the bondinginterface between pedestal portions 72, 74 and internal pendulum 64 isminimized by the inherent symmetry of the design.

Pedestal portions 72, 74 are sized according to known design principlesto provide adequate bond area to provide for proper alignment of covers68, 70 and adequate bond strength in the specific application.Alternatively, pedestal portions 72, 74 may be formed on the opposingsurfaces of internal pendulum 64 or may be discrete mechanicalcomponents.

Internal pendulum 64 is constrained to travel along an input axis 78substantially perpendicular to the plane of internal pendulum 64 by, forexample, providing flexure 66 with sufficient lateral stiffness topreclude motion in the plane of internal pendulum 64. Shock stop anddamping functions are performed in the configuration of FIG. 4 bysandwiching frame 62 between top and bottom covers 68, 70. Thus, travelof internal pendulum 64 along input axis 78 is physically limited bycovers 68, 70 contacting frame 62. Squeeze film damping is provided byproviding equivalent gaps between stationary mechanism frame 62 and topand bottom covers 68, 70 and introducing a pressurized fluid into thegaps. Sensor 60 is mounted in an accelerometer housing 76, representedby ground, by mounting frame 62 to housing 76 using conventionalmounting means, for example, by epoxy bonding.

The embodiment illustrated in FIG. 5 maximizes isolation from externalerror sources and maximizes active reaction mass while optimizing heatflow. The embodiment illustrated in FIG. 5 reverses the traditionalroles of the elements. In this embodiment, the sensor is centrallymounted at the center of the structure which would form the pendulum ina traditional sensor. The covers are connected to the externalsupporting frame of the reaction mass. Thus, the structure which wouldtraditionally form the pendulum instead forms the fixed mountingstructure while the external pendulum portion combines with the coversto form the active reaction mass. This embodiment further providessealing of the mechanism by fixing the external pendulum member to thecovers. The combination of the external pendulum member and the coversprovides an increased active reaction mass considerably in excess of thereaction mass of which the reaction mass was previously capable.

Thus, the embodiment of FIG. 5 provides substantially reduced mechanismsize and maximum active mass while maximizing isolation from externalerror sources and optimizing heat flow. In sensor 80, an externalreaction mass 82 includes an external pendulum portion 84 fixed to antop cover 86 and a bottom cover 88. External reaction mass 82 issuspended by flexure 90 from internal frame member 92. External pendulumportion 84 is formed with an internal passage through its thicknesswherein internal frame member 92 is nested. Some or all of externalpendulum 84, internal frame member 92 and flexural suspension member 90may be formed of a single substantially planar substrate usingmanufacturing techniques known to those of skill in the art. Thesubstrate material may be, for example, quartz, silicon or anothersuitable material.

Internal frame member 92 may include an isolation feature, for example,one of the isolation systems described in connection with the embodimentof FIG. 3, above. External reaction mass 82 encloses internal framemember 92 in a chamber comprising a substantially planar externalpendulum 84 sandwiched between top cover 86 and bottom cover 88. Top andbottom covers 86, 88 are fixed to opposing sides of external pendulum 84using, for example, conventional epoxy bonding techniques, thus formingthe top, bottom and side walls of a chamber which surrounds andsubstantially encloses internal frame member 92. For example, top andbottom covers 86, 88 may be bonded to external pendulum 84 of externalreaction mass 82 along the periphery of external pendulum 84. Thus, topcover 86 and bottom cover 88 add their mass to the mass of externalpendulum 84 which moves essential active reaction mass to externalreaction mass 82 and maximizes the reaction mass without increasingsensor dimensions. External reaction mass 82 is constrained to movementalong an input axis 98 substantially perpendicular to the plane ofexternal pendulum 84 of external reaction mass 82 by flexure 90. Thus,an input force, for example, an acceleration input, applied along inputaxis 98 displaces external reaction mass 82 a distance, x, along inputaxis 98. Vibrating beam force sensors 28, 30 are mounted to extendbetween internal frame member 92 and external pendulum 84 of externalreaction mass 82 such that displacement of external reaction mass 82imparts either a compressive or a tensile force to vibrating beam forcesensors 28, 30.

A central mounting pedestal 94 provides a connection between internalframe member 92 and the accelerometer housing 96, represented by ground.Central pedestal mount 94 is formed with two substantially parallelopposing surfaces. Central pedestal mount 94 passes through a passageformed in bottom cover portion 88 of external reaction mass 82 and oneend is fixed to internal frame member 92 by, for example, conventionalepoxy bonding techniques known to those of skill in the art. Theopposing end of central pedestal mount 94 is fixed to accelerometerhousing 96 by appropriate means. Alternatively, central pedestal mount94 may be formed in an appropriate surface of housing 96. Centralpedestal mount 94 is sized according to known design principles toprovide adequate bond area to achieve proper alignment of externalreaction mass 82 relative to housing 96 and adequate bond strength tosupport sensor mechanism 80 in the specific application. Thus, theembodiment of the present invention illustrated in FIG. 5 provides anincreased pendulous arm length by maximizing the dimensions of activeexternal reaction mass 82 and positioning active external reaction mass82 external to internal frame member 92. Thus, the available mass isused more efficiently.

The embodiment of FIG. 5 also results in increased isolation fromexternal stresses, including mounting stresses, by providing localizedstrain coupling instead of multiplying external strain coupling acrossthe length of the mechanism. The isolation function of externalisolation features, for example, isolation slots 32, 34 as illustratedin FIG. 2, is obviated. Rather, the isolation function is performed bycentral pedestal mount 94. Central pedestal mount 94 isolates the sensormechanism from external strains by reducing the interface area to aminimum and by placing the interface point at the center of sensormechanism 80 such that the moment arm over which any strain-inducedforce acts is also reduced to a minimum. Thus, stress magnitude isminimized and constrained to a small locality. The stressed locality isnearly ideal because it is centrally located and symmetrical relative tothe vibrating beam force sensors. Strain-induced forces and interfaceforces may be further reduced by fixing central pedestal mount 94 tointernal frame member 92 using compliant epoxy bonding techniques, forexample, the techniques discussed in connection with the embodiment ofFIG. 3, above. Additionally, central pedestal mount 94 may be formed ofthe same material used to manufacture internal frame member 92 such thatthe thermal expansion coefficients of the two structures match exactlyand heat distortion of central pedestal mount 94 does not induce thermalstrain at the interface with internal frame member 92. Alternatively,central pedestal mount 94 may be formed on an appropriate surface ofaccelerometer housing 96 or on an appropriate surface of internal framemember 92 whereby protection from thermally-induced strain may be tradedagainst potentially lower manufacturing costs.

The embodiment of the present invention illustrated in FIG. 5 furtherprovides improved thermal response. The heat capacity of suspendedexternal reaction mass 82 is lower than that of typical sensor designsbecause the internal mount/external reaction mass configuration providesgreater pendulousity using less total pendulous mass. Thus, the rawthermal response time is quicker than in conventional sensor designs dueto the higher mass efficiency of the reaction mass. Further thermalresponse aid results from the centrally localized mounting. Because theheat source is centrally localized by central pedestal mount 94, heatflow paths are simplified. The heat flow paths are nearly symmetricaland easily controlled by conventional methods which allows more precisethermal ramp matching. The improved heat flow paths combined with thereduced heat capacity in the reaction mass produces smaller, morequickly dissipated heat flow-induced errors.

Isolation features may be added around the central pedestal mount 94 tofurther decouple stress and control heat flow. The isolation featuresmay be provided in internal frame member 86 and may be formed, forexample, using one of the isolation systems discussed in connection withthe embodiment of FIG. 3, above. Any isolation features may be muchsmaller than disclosed in the prior art and still prove more effectiveagainst the lower initial stress levels because the internalmount/external reaction mass configuration decouples stress and controlsheat flow. Thus, simple isolation features known to those of skill inthe art will dissipate both stress and direct heat flow. However, asnoted above, isolation features are outside the scope of this inventionand are not shown.

The shock stop and damping functions are performed in the embodiment ofthe present invention as illustrated in FIG. 5 by sandwiching internalframe member 92 between top and bottom covers 86, 88. Thus, travel ofexternal reaction mass 82 along input axis 98 is physically limited bycontact between covers 86, 88 and opposing sides of fixed internal framemember 92. Squeeze film damping is provided by providing equivalent gapsbetween stationary internal frame member 92 and top and bottom coverportions 86, 88 and introducing a pressurized gas into the gaps.

Alternatively, the present invention according to the embodimentillustrated in FIG. 5 may use an external reaction mass 82 whichoccupies less than the volume formerly occupied by sensor 10. Thus, anacceleration sensor having an active reaction mass commensurate withthat of the configuration illustrated in FIGS. 1 and 2 occupies lessvolume than if configured according to typical sensor designs. Those ofskill in the art will recognize that present invention as embodied inthe configuration of FIG. 5 provides an opportunity to trade volume forpendulousity and vice versa along a continuum ranging from maintainingthe original active reaction mass in a reduced volume sensor toincreasing the active reaction mass to fill the original volume. Thoseof skill in the art will further recognize that the degree to whichvolume is traded for active reaction mass is dependent on many designfactors including, for example, available space in which to mount theacceleration sensor, the g-range required by the application, and thematerials used in manufacturing the sensor.

The present invention resolves the manufacturing cost issues presentedby conventional sensor designs by providing the inventive features atessentially no additional manufacturing cost. Neither additionalcomponents nor additional processing are required to practice thepresent invention. The accelerometer topology of the present inventionsimply reconfigures previously static cover mass to an active condition.

Preferred embodiments of the invention have been described. Those ofskill in the art will recognize that many alternative embodiments of thepresent invention are possible. In many alternative embodiments of thepresent invention the effective mass center can be placed at a largerradius from the flexures. Thus, the pendulousity increase can be evengreater than the active mass increase. Similarly, the effective centerof damping can also be moved to a larger radius from the flexures toprovide greater damping using a smaller area.

Those of skill in the art will recognize that the present invention canbe applied to various types of accelerometers utilizing a reaction massincluding, but not limited to, vibrating beam accelerometers, capacitiveaccelerometers, capacitive rebalance accelerometers, and translationalmass accelerometers. For at least these reasons, the invention is to beinterpreted in light of the claims and is not limited to the particularembodiments described herein.

I claim:
 1. A pendulous accelerometer comprising:a housing; a framefixed to said housing, said frame including first and secondsubstantially planar covers and a substantially planar internal framemember, said first cover substantially parallel with and spaced awayfrom said second cover and said internal frame member suspended betweensaid first and second covers; a first pedestal portion extending betweena first side of said internal frame member and a first side of saidfirst cover; a second pedestal portion extending between a second sideof said internal frame member and a first side of said second cover; areaction mass rotatably attached external to said internal frame member;and means for measuring a displacement of said reaction mass.
 2. Thependulous accelerometer of claim 1 wherein said reaction mass includes apassage formed therein.
 3. The pendulous accelerometer of claim 2wherein said internal frame member is nominally disposed within saidpassage of said reaction mass and further comprising a flexuralsuspension member extending between said reaction mass and said internalframe member for rotatably connecting said reaction mass to saidinternal frame member.
 4. The pendulous accelerometer of claim 3 whereinsaid displacement measuring means comprises first and second vibratingbeam force sensors extending between said internal frame member and saidreaction mass.
 5. A method of manufacturing a pendulous accelerometer,comprising the steps of:forming a frame comprising first and secondsubstantially planar covers and a substantially planar internal framemember, said first cover formed substantially parallel with and spacedaway from said second cover and said internal frame member suspendedbetween said first and second covers; forming a first pedestal portionextending between a first side of said internal frame member and a firstside of said first cover; forming a second pedestal portion extendingbetween a second side of said internal frame member and a first side ofsaid second cover; fixing said frame to a housing; mounting a reactionmass external to said internal frame member and rotatably attached tosaid internal frame member; and mounting a detector between saidinternal frame member and said reaction mass, said detector measuring adisplacement of said reaction mass relative to said internal framemember.
 6. The method of claim 5 wherein said reaction mass mountingstep includes fixing a cover to said housing and fixing an internalframe member to said cover.
 7. The method of claim 6 wherein saidreaction mass and said internal frame member are formed of a singlesubstrate.